Oops, seems my examples were treated as code! Lemme try again:

I often compose messages in a plain-text editor (e.g., Notepad++, Atom) and paste into a new Thunderbird message.

Thunderbird converts line breaks into -br- tags; if I use two 'returns' between lines, I get br br (with the angle brackets, of course). This is regardless of my setting for "Use Paragraph format instead of Body Text by default."

Is there any way to get Thunderbird to recognize two plain-text returns as a paragraph break, so I get p tags instead? I've tried different line breaks in my editor (CR LF, CR, and LF) and different encodings (UTF-8, ANSI), but no joy.

Thanks for any suggestions!

You need to get html into a plain text message. Paste into Write window. Highlight all the content. 'Insert' > 'HTML' a small window opens, note you can extend window. Where you see the two < br > < br > line breaks, in the first one, edit the < br > so it says < p > and delete the second one. Work through text. Click on 'Insert'

Obviously there isn't a space between <>, but it's the only way I can get the chevrons to display :)

But why are you wanting to do all this extra work when you can do exactly the same - just type email - in a new Write message as if it were the same as using Notepad and then save as draft if you have not finished it. I guess it is just something you've done for a while.